HARMONICS PRODUCED BY VARIABLE-SPEED FAN SYSTEMS

The characteristics of electric power harmonics generated by four successive designs of a 41 cm diameter, variable-speed fan system used for ventilation of agricultural buildings were investigated Harmonic analyzers were programmed to calculate harmonic frequencies of the 2nd through the 63rd orders at several motor control voltages during tests of each fan design. Alterations to the fan system included changes to the motor, the impeller, and the controller. Motors used in this study were single-phase 0.25 kW and 0.20 kW. The 230 Vac fan motor produced greater total harmonic distortion with a triac-based controller in the circuit to reduce motor voltage when compared with no controller. The third harmonic was the greatest contributor to total harmonic distortion of current and voltage. The magnitudes of even order harmonics were negligible, and odd harmonics decreased with increasing order. Fan static pressure had relatively little effect (less than 10%) on harmonic levels. Heavier motor loading and smaller motor capacitors tended to lower the generation of harmonics from the fan system. Comparisons of harmonic generation were made between triac and transformer-based controllers. Also, voltage harmonics before and after a triac-based controller were evaluated.